1. Field of the Invention
The present invention pertains to the field of burners, particularly burners for use in gas turbines. It relates to an appliance for, and a method of, operating a burner, in which a combustion air flow transports fuel into a combustion chamber where the fuel is burnt.
2. Discussion of Background
In modern burners, particularly in burners such as are used in gas turbines, it is becoming increasingly important to keep the combustion both as efficient as possible and as free from pollutants as possible. Pollutant limits are specified by the authorities, inter alia, and the regulations with respect to CO and NOx emission are becoming increasingly strict. The corresponding optimization of the combustion can take place in a variety of ways, for example by the admixture of additives such as water to the fuel, by the employment of catalyzers or also by ensuring ideal fuel/air mixtures for the combustion. Optimum fuel/air ratios can be achieved by premixing fuel and combustion air (so-called premixing burners) or by injecting fuel and combustion air together in a special manner into the combustion space.
EP-B1-0 321 809 reveals a burner for liquid and gaseous fuels, without premixing section, in which combustion air supplied externally enters through at least two inlet slots tangentially between hollow half-cones in an offset arrangement and, in this location, flows in the direction of the combustion chamber, and in which the liquid fuel is injected centrally on the tapered side, facing away from the combustion chamber, of the half-cones. The fuel is therefore entrained and xe2x80x9cenvelopedxe2x80x9d, so to speak, by the combustion air, so that a conical liquid fuel profile forms between the half-cones, spreads out in the direction of the combustion chamber and burns there. Gaseous fuel is injected transversely into the entering air, through rows of holes, from fuel supply pipes which extend along the air inlet slots.
A problematic feature of such burners, and generally in the case of burners in which a flow of combustion air flows in a similar manner into a combustion chamber, is the emergence of the combustion air in the combustion chamber. Whereas the combustion air in the burner slides along the walls of the half-cones and is guided by them, a shear layer forms immediately behind the front edge of the half-cones, in the flow direction of the combustion air. This shear layer is located between the substantially stationary and hot combustion gases located in the combustion chamber and the emerging, flowing mixture of fuel and combustion air. Now, it is in the nature of such shear layers that they roll up at some point and result in vortices. They roll up in such a way that so-called Kelvin-Helmholtz waves, whose wave crests extend transversely to the flow direction, form first on the shear layers and then generate vortices.
It is found that it is these instabilities on shear layers, in combination with the combustion process taking place, which are mainly responsible for an important class of thermoacoustic oscillations initiated by reaction rate fluctuations. These substantially coherent waves lead, in the case of a burner of the type mentioned above and at typical operating conditions, to vibrations with frequencies of approximately 100 Hz. Since this frequency coincides with typical fundamental natural modes of many gas turbine annular burners, the thermoacoustic oscillations present a problem.
Accordingly, one object of the invention is to provide a novel appliance or burner and a method which prevents the formation of coherent flow instabilities of the combustion air flow after emergence into the combustion chamber.
This object is achieved in an apparatus and a method of the type described at the beginning by perturbation air being injected into the combustion air flow. The core of the invention therefore consists in the fact that the injected perturbation air already prevents the excitation of thermoacoustic oscillations in a specific manner at the cause of their formation.
A first preferred embodiment of the invention is one wherein the coherent flow instabilities, after emergence of the combustion air into the combustion chamber, form as a consequence of shear layers between the combustion air flow and substantially stationary hot gases in the combustion chamber, and wherein the perturbation air acts on these shear layers. The perturbation air is then preferably injected into the combustion air flow substantially at right angles to a main flow direction of the combustion air flow and substantially parallel to the shear layers, preferably even into the shear layers. By this means, the formation of Kelvin-Helmholtz waves in the flow direction is specifically nipped in the bud.
Another embodiment of the invention is one wherein the burner is a double-cone burner, wherein the injection of the perturbation air takes place through perturbation nozzles, and wherein the perturbation air occurs directly at the front edges of the half-cones, where the shear layers form. If, furthermore, the perturbation nozzles are preferably distributed uniformly at certain distances apart around the peripheries of the front edges of the half-cones, this perturbs the periodicity of the waves on the shear layers and specifically prevents the thermoacoustic oscillations at the outset of their formation.
Further embodiments of the method and of the apparatus follow from the dependent claims.